Activation entropy helps explain anomalous flow stress temperature dependence in copper

Abstract

Thermal activation of dislocations is critical for predicting the mechanical response of materials under common experimental conditions. According to transition state theory (TST), the rate for the system to overcome free energy barriers depends on an attempt frequency, activation free energy, and temperature. We computed the rate for edge and screw dislocation dipoles to overcome their interaction fields at various temperatures, Langevin friction coefficients, and shear stresses using Molecular Dynamics (MD), Schoecks entropy formalism and compared with Kramers rate theory. Kramers theory matches the rates computed dynamically, which depend on Langevin friction, increasing with weaker friction. Statically, using Schoeck formalism to compute the entropy along the minimum energy path (MEP), we found significant entropic effects that lead to an increase of the critical resolved shear stress with temperature and could help explain the long-standing anomaly observed at low to intermediate temperatures in copper and other metals, where the flow stress increases with temperature.